Switched capacitor amplifier with higher gain and improved closed-loop gain accuracy

ABSTRACT

A switched capacitor CMOS amplifier uses a first stage non-inverting CMOS amplifier driving a second stage inverting CMOS amplifier. The first stage amplifier is provided with positive feedback to substantially increase the gain of the first stage amplifier. In the described examples, the positive feedback is provided either by connecting a capacitor from the output to the input of the first stage amplifier or by connecting a shunt transistor in parallel with an input transistor and driving the transistor from the output of the first stage amplifier. The substantially increased gain resulting from the positive feedback allows the gain of the switched capacitor amplifier to be set by the ratio of the capacitance of an input capacitor to the capacitance of a feedback capacitor. The amplifier also includes switching transistors for periodically discharging the input capacitor and the feedback capacitor.

TECHNICAL FIELD

This invention relates to amplifier circuits, and, more particularly, toa switched capacitor amplifier circuit that provides higher open-loopgain and improved closed-loop gain accuracy.

BACKGROUND OF THE INVENTION

Switched capacitor amplifiers are compatible with CMOS technology andconsequently are therefore frequently used as analog building blocks inCMOS circuits. In general, the design methodology in CMOS amplifiersassumes the use of infinite gain and infinite bandwidth operationalamplifiers. However, CMOS amplifiers have a relatively low gain becauseof the low gain inherent in CMOS devices. The maximum gain of a CMOSswitched amplifier, i.e., open circuit gain, is approximately 25, andmay be as low as 10. The low gain of CMOS switched amplifiers introducefinite gain error when the gain of the amplifier is assumed to be theratio of the capacitance of an input capacitance to the capacitance of afeedback capacitor.

FIG. 1 shows a switched capacitor amplifier 100 having an inputcapacitor 104 having a capacitance of Cin connected to an invertinginput 116 of an operational amplifier 112, which is assumed to haveinfinite gain. The amplifier 100 also includes a feedback capacitor 108having a capacitance of Cfb coupled in series with an NMOS switchingtransistor 110 between an output 124 of the amplifier 112 and theinverting input 116. The feedback capacitor 108 forms a closed loop viathe transistor 110 to provide feedback from an output terminal 124 ofthe operational amplifier 112 to the inverting input terminal 116. Anon-inverting input 120 of the operational amplifier is connected to theground. As a matter of convention, it should be understood that theterms “non-inverting input” terminal and “inverting input” terminal areused with respect to their relationship to a particular output terminal.An amplifier could alternatively be considered to have an “invertingoutput” terminal and a “non-inverting output” terminal, as one skilledin the art will appreciate. For example, rather than refer to anamplifier as having an inverting input terminal and an output terminal,one could refer to the same amplifier as having an input terminal and aninverting output terminal.

Another switched NMOS transistor 126 is connected between the inputcapacitor 104 and an input voltage source 128. The gates of thetransistors 110, 126 both receive a Q₁ switching signal so they are bothON at the same time. When the transistors 110, 126 are turned ON, theinput voltage source 128 is applied to the input capacitor 104. As aresult, the input capacitor 104 is charged since the input terminal 116is a virtual ground because of the feedback through the capacitor 108.The capacitor 108 is also charged for that same reason. The capacitor108 is charged to a voltage Vout that is equal to the product of thevoltage −Vin and ratio of the capacitance of the input capacitor 104 tothe capacitance of the feedback capacitor 108.

A switched NMOS transistor 136 is connected between the input capacitor104 and the ground, another NMOS transistor 138 is connected between thefeedback capacitor 108 and ground, and another NMOS transistor 140 isconnected between the output terminal 124 and the inverting input 116.When the transistors 136, 138, 140 are ON responsive to a high Q₂ signalapplied to their gates, the capacitors 104, 108 are discharged to theground, and the output terminal 124 is reset to zero volts.

In operation, the Q₁ and Q₂ signals are alternately driven to a highlogic level. Therefore, the transistors 110, 126 are operated in acomplementary manner with the transistors 136, 138, 140 thereby causingthe capacitors 104, 108 to be alternately charged and discharged.Periodically discharging the capacitors 104, 108 prevents offsets thatwould otherwise be present at the output terminal 124 of the amplifier100.

In the discussion of the amplifier 100 shown in FIG. 1, it was assumedthat the open-loop gain of the operational amplifier 112 was infinite.However, a typical CMOS differential amplifier does not have anopen-loop gain that even approaches infinity. With an operationamplifier 112 having a more limited open-loop gain, the approximateclosed loop gain of the operational amplifier is given by the followingequation:Vout/Vin=−Av/[((1+Cfb*(Av+1))/Cin)]  (1)where Av is the open-loop gain of the operational amplifier.If Av is very large, equation (1) can be approximated as follows:Vo/Vin=−Cin/Cfb  (2)

Thus, as explained above with respect to the amplifier 112 used in theamplifier 100 of FIG. 1, if the open-loop gain Av is very large, theclosed-loop gain of the amplifier is approximately equal to the ratio ofthe capacitance Cin of the input capacitor 104 to the capacitance Cfb ofthe feedback capacitor 108. However, since CMOS amplifiers invariably donot have high open-loop gain, equation (2) does not provide an accurateresult.

Suppose for example, Av=100 and Cin/Cfb=10. If Av is very large,equation (2) can be used, and Vo/Vin=−10. However, if Av is 10, then,equation (1) provides Vo/Vin =−9. The simplified formula, i.e., equation(2), based on the ratio of the capacitances predicts a gain of 10, butthe actual gain from a more accurate analysis using equation (1)predicts a gain of 9. The error, which is the difference in gain, iscaused by the low open-loop gain of the CMOS amplifier. If the open-loopgain of the CMOS amplifier could be increased, the error could beeliminated, and the closed-loop gain of the amplifier would be simplythe ratio of the input capacitance Cin to the feedback capacitance Cfbgiven by equation (2). Since the capacitance of capacitors can becontrolled fairly precisely during manufacture, the gain of a switchedcapacitance amplifier could then be precisely controlled.

Another technique for dealing with the relatively low open-loop gain ofCMOS amplifiers is to factor the open-loop gain of the CMOS amplifierinto the closed-loop gain using equation (1) to provide the desiredlevel of gain. However, it is fairly impractical to fabricate a CMOSamplifier with a precisely controlled open-loop gain since the gain canvary with process variations. The open-loop gain of a CMOS amplifier canalso change with temperature or supply voltage variations. Without astable value for the open-loop gain of a CMOS amplifier, it is notpossibly to use equation (1) to calculate a precise closed-loop gain fora switched capacitor amplifier.

There are also other approaches that can be used for attempting toprovide switched capacitance CMOS amplifiers with stable gaincharacteristics. However, all of these approaches impose limitations orcosts on switched capacitance CMOS amplifiers using these approaches.For example, some approaches result in the use of greatly increasedsurface area on a die, and other approaches provided somewhat limitedperformance.

Accordingly, there is a need for a CMOS amplifier circuit having veryhigh open-loop gain so that the closed-loop gain of a switched capacitoramplifier can be precisely controlled and does not vary with process,supply voltage and temperature variations.

SUMMARY OF THE INVENTION

An amplifier circuit includes a first stage amplifier having an inputterminal and an output terminal. The first stage amplifier is configuredto operate with positive feedback and therefore has a very high gain.The amplifier circuit also includes a second stage amplifier having aninput terminal and an output terminal. The input terminal of the secondstage amplifier is coupled to the output terminal of the first stageamplifier. The first stage amplifier and the second stage amplifiertogether forming an inverting amplifier between the output terminal ofthe second stage amplifier and the input terminal of the first stageamplifier. The second stage amplifier may have a relatively small amountof gain compared to the gain of the first stage amplifier. A firstfeedback capacitor is connected between the output terminal of thesecond stage amplifier and the input terminal of the first stageamplifier. The first feedback capacitor provides negative feedback fromthe output terminal of the second stage amplifier to the input terminalof the first stage amplifier. The positive feedback of the first stageamplifier may be provided by a capacitor connected between anon-inverting output of the first stage amplifier and the input of thefirst stage amplifier. Positive feedback may also be provided byconnecting a transistor in parallel with an input transistor having agate that is coupled to the input terminal of the first stage amplifier.The gate of the transistor is then coupled to a non-inverting outputterminal of the first stage amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one example of a conventional switchedcapacitor amplifier.

FIG. 2 is a schematic diagram of a CMOS switched capacitor amplifieraccording to one example of the invention.

FIG. 3A is a schematic diagram of a CMOS amplifier that can be used inthe switched capacitor amplifier of FIG. 2 or in some other example ofthe invention.

FIG. 3B is a schematic diagram of an equivalent circuit for the CMOSamplifier of FIG. 3A.

FIG. 4 is a schematic diagram of another example of a CMOS amplifierthat can be used in the switched capacitor amplifier of FIG. 2 or insome other example of the invention.

FIG. 5 is a schematic diagram of another example of a CMOS amplifierthat can be used in the switched capacitor amplifier of FIG. 2 or insome other example of the invention.

FIG. 6 is a schematic diagram of a CMOS switched capacitor amplifieraccording to another example of the invention.

DETAILED DESCRIPTION

A switched capacitance CMOS amplifier 200 according to one example ofthe invention is shown in FIG. 2. The amplifier 200 uses a first CMOSamplifier 210, a second CMOS amplifier 220, and the same components thatwere used externally to the amplifiers 210, 220 that were usedexternally of the amplifier 112 in the amplifier 100 shown in FIG. 1. Inaddition, the first CMOS amplifier 210 includes a capacitor 224 having acapacitance of Cc connected between its output 230 and a non-invertinginput 234. An inverting input of the 236 of the amplifier 210 isconnected to ground. In operation, the capacitor 224 provides theamplifier 210 with positive feedback, thereby greatly increasing itsgain.

The output 230 of the first CMOS amplifier 210 is connected to aninverting input 240 of the second CMOS amplifier 220. A non-invertinginput 242 of the amplifier 220 is connected to ground. The amplifier220, like typical CMOS amplifiers, has a relatively low gain. However,because of the very high gain of the first amplifier 210, the gain ofthe two amplifiers 210, 220 together is very large. The amplifiers 210,220 can be considered to be a single amplifier having a very largeopen-loop gain as in the amplifier 100 shown in FIG. 1. The amplifier200 therefore operates in the same manner as explained above for theamplifier 100, and its closed-loop gain is therefore given by equation(2) as simply −Cin/Cfb, where Cin is the capacitance of the inputcapacitor 104 and Cfb is the capacitance of the feedback capacitor 108.As previously explained, it is possible to fabricate the capacitors 104,108 with fairly precise capacitances. Furthermore, the capacitances ofthese capacitors do not change appreciably with process, supply voltageand temperature variations. As a result, the amplifier 200 has precise,very stable gain characteristics.

A CMOS amplifier 250 with positive feedback according to one example ofthe invention is shown in FIG. 3A. The amplifier includes a pair ofdifferential NMOS input transistors 254, 256, a current sink NMOStransistor 258, and a pair of PMOS load transistors 261, 262 coupled toeach other to act as a current mirror. The transistors 254-262 arecoupled to each other in a conventional manner, and such amplifiers arein common use. The gate of the input transistor 254 serves as anon-inverting input terminal 260 to which an input voltage Vin iscoupled through an input capacitor 264 having a capacitance of Cin. (TheNMOS switching transistors shown in FIGS. 1 and 2 have been omitted fromFIG. 3A in the interest of clarity). The gate of the input transistor256 serves as an inverting input terminal 266, which is connected toground. The transistors 254, 260 form a first current path, and thetransistors 256, 262 form a second current path. The current mirrorformed by the transistors 260, 262 ensures that the currents through thefirst and second current paths are equal to each other. The drain of theinput transistor 256 serves as an output terminal 268 for the amplifier250. A feedback capacitor 270 having a capacitance of Cc is connectedbetween the output terminal 268 and the non-inverting input terminal260. The feedback capacitor 270 provides positive feedback to greatlyincrease the gain of the amplifier 250.

An equivalent circuit for the CMOS amplifier 250 of FIG. 3A is shown inFIG. 3B. The voltage between the input terminals 260, 266 is labeled Vx,and the voltage at the output terminal 268 is Vo. The input transistor256 is modeled by a current source 274 providing a current having amagnitude of gm*Vx, where gm is the transconductance of the amplifier250. The transistor 262 is modeled by a load resistor 276 having aresistance R_(L). A second current source 278 provides a relativelysmall current that can be ignored for the present analysis.

Without the presence of the feedback capacitor 270, the voltage Vx wouldbe equal to the input voltage Vin. The voltage Vo would therefore be theproduct of the current gm*Vin and the resistance R_(L) of the loadresistor 276, i.e., Vin*gm*R_(L). The gain of the amplifier 250, VoNin,would therefore be simply gm*R_(L).

With the feedback capacitor 270, the gain of the amplifier 250 is givenby the equation:Vo/Vin=(gm*R _(L) *Cin)/[Cin−Cc(gm*R _(L)−1)]  (3)

It can be seen from Equation 3 that the gain VoNin can become very largeif the denominator Cin−Cc(gm*R_(L)−1) becomes very small by making Cinonly slightly larger than Cc(gm*R_(L)−1). However, the amplifier 250 isconditionally stable and will not oscillate as long as the value ofCin−Cc(gm*R_(L)−1) does not become too large. Nevertheless, gains of 100or more are easily achievable.

An alternative example of a CMOS amplifier 280 that can be used in theswitched capacitor amplifier of FIG. 2 or in some other example of theinvention is shown in FIG. 4. The amplifier 280 can be thought of as thecompliment to the amplifier 250 shown in FIG. 3A in that it uses NMOSload transistors 282, 284 instead of the PMOS load transistors 261, 262used in the amplifier 250, and it uses PMOS input transistors 286, 288and a PMOS current source transistor 290 instead of the NMOS inputtransistors 254, 256 and NMOS current sink transistor 258, respectively,used in the amplifier 250. However, the amplifier 280 operates insubstantially the same manner as the amplifier 250, and it uses the sameinput capacitor 264 and the same feedback capacitor 270.

Still another example of a CMOS amplifier 300 that can be used in theswitched capacitor amplifier of FIG. 2 or in some other example of theinvention is shown in FIG. 5. Like the amplifier 250 shown in FIG. 3A,the amplifier 300 uses a pair of PMOS load transistors 304, 306connected to each other as current mirrors. The amplifier 300 also usesa pair of NMOS input transistors 310, 312 and an NMOS current sinktransistor 316. However, unlike the amplifier 250, in which the outputterminal 268 is taken from the drain of the transistor 312, an outputterminal 318 is taken from the drain of the transistor 310. As a result,the gate of the transistor 310 constitutes an inverting input ratherthan a non-inverting input as in the amplifier 250 of FIG. 3A. In theamplifier 300 of FIG. 5, positive feedback is provided by connecting anNMOS transistor 320 in parallel with the inverting input transistor 310.The transistor 320 preferably has a small channel width in comparison tothe channel width of the transistor 310. The gate of the transistor 320is driven by the drain of the input transistor 312.

In operation, an increase in the magnitude Vin of the input voltagedecreases the impedance of the input transistor 310, thereby decreasingthe voltage at the drain of the transistor 310. Consequently, themagnitude Vo of the output voltage decreases. The decreased impedance ofthe input transistor 310 also causes more current to flow through thefirst current path formed by the transistors 304, 310. However, becauseof the current mirror, the current flowing through the second currentpath formed by the transistors 306, 312 must decrease. The impedance ofthe transistor 306 is essentially constant. As a result, the decreasedcurrent flowing through the second current path increases the voltage atthe drain of the non-inverting input transistor 312, which is coupled tothe gate of the transistor 320. The impedance of the transistor 320 thendecreases to further decrease the impedance across the input transistor310, which further decreases the magnitude Vo of the output voltage.Consequently, the transistor 320 provides the amplifier 300 withpositive feedback.

A specific example of a switched capacitor CMOS amplifier 340 is shownin FIG. 6. The amplifier 340 uses as its first amplifying stage thepositive feedback CMOS amplifier 250 shown in FIG. 3A. The amplifier 250functions in the same manner as previously explained. Therefore, thecomponents have been provided with the same reference numerals, and, inthe interest of brevity, an explanation of their function and operationwill not be repeated.

The amplifier 340 includes as its second amplifying stage a unity gaininverting amplifier 344 formed by a PMOS input transistor 348 connectedin series with a diode-connected NMOS transistor 350. The non-invertingoutput terminal 268 of the amplifier 250 is connected to the gate of thetransistor 348, and an output terminal 354 is taken at the drain of thetransistor 348. As in the other examples, an input capacitor 360 havinga capacitance of Cin is connected to the gate of the transistor 254, anda feedback capacitor 364 having a capacitance of Cfb is connectedbetween the output terminal 354 and the non-inverting input terminal260. Insofar as the amplifier 344 is an inverting amplifier, thecapacitor 364 provides negative feedback. The gain of the amplifier 250is given by equation (3) and, since the gain of the amplifier 344 issimply −1, the open-loop gain of the amplifier is given by the equation:Vo/Vin=−(gm*R _(L) *Cin)/[Cin−Cc(gm*R _(L)−1)]  (4)

As previously explained with reference to FIG. 3A, the gain of theamplifier 250 can be made very large, thereby making the open-loop gainof the amplifier 340 very large. Consequently, the closed-loop gain ofthe amplifier 340 is essentially equal to Cin/Cc, where Cin is thecapacitance of the input capacitor 360 and Cc is the capacitance of thefeedback capacitor 364. The closed-loop gain of the amplifier 340 istherefore substantially insensitive to process, supply voltage andtemperature variations.

In addition to the first stage amplifier 250, the unity gain invertingamplifier 344, the input capacitor 360, and the feedback capacitor 364,the switched capacitor CMOS amplifier 340 shown in FIG. 6 uses the samecomponents that were used externally of the amplifier 112 in theamplifier 100 shown in FIG. 1. These components operate in the samemanner responsive to the Q₁ and Q₂ signals to periodically charge anddischarge the input capacitor 360 and the feedback capacitor 364.However, these components have been omitted from FIG. 6 in the interestof clarity.

Although the present invention has been described with reference to thedisclosed embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For example, the amplifier 340 of FIG. 6uses the unity gain inverting amplifier 344 as the second amplifyingstage of the amplifier 340 because the non-inverting positive feedbackamplifier 250 is used as the first amplifying stage of the amplifier340. However, if the inverting positive feedback amplifier 300 shown inFIG. 5 was used as the first amplifying stage of the amplifier 340, anon-inverting amplifier would be used as the second amplifying stage ofthe amplifier 340. Such modifications are well within the skill of thoseordinarily skilled in the art. Accordingly, the invention is not limitedexcept as by the appended claims.

1. An amplifier circuit, comprising: a first stage amplifier having aninput terminal and an output terminal, the first stage amplifier beingconfigured to operate with positive feedback; a second stage amplifierhaving an input terminal and an output terminal, the input terminal ofthe second stage amplifier being coupled to the output terminal of thefirst stage amplifier, the first stage amplifier and the second stageamplifier together forming an inverting amplifier between the outputterminal of the second stage amplifier and the input terminal of thefirst stage amplifier; and a first feedback capacitor connected betweenthe output terminal of the second stage amplifier and the input terminalof the first stage amplifier, the first feedback capacitor beingoperable to provide negative feedback from the output terminal of thesecond stage amplifier to the input terminal of the first stageamplifier.
 2. The amplifier circuit of claim 1 wherein the first stageamplifier comprises: an input capacitor connected between an amplifierinput terminal and the input terminal of the first stage amplifier; anda second feedback capacitor connected between the output terminal of thefirst stage amplifier and a non-inverting input terminal of the firststage amplifier, the second feedback capacitor being operable to providepositive feedback to the first stage amplifier.
 3. The amplifier circuitof claim 1 wherein the first stage amplifier comprises: an inputtransistor having a gate coupled to the input terminal of the firststage amplifier; and a positive feedback transistor coupled in parallelwith the input transistor, the positive feedback transistor having agate coupled to a node of the first stage amplifier that isnon-inverting with respect to the input signal applied to the inputterminal of the first stage amplifier.
 4. The amplifier circuit of claim1 wherein the first stage amplifier comprises: a first and a second CMOStransistors coupled to form a first path for a current to flow from afirst potential, the second CMOS transistor having a gate coupled to theinput terminal of the first stage amplifier; a third and a fourth CMOStransistors coupled to form a second path for a current to flow from thefirst potential, the fourth CMOS transistor having a gate coupled to afirst bias voltage, the first and third CMOS transistors being coupledtogether at their respective gates; and a fifth CMOS transistor coupledto the second and fourth CMOS transistors and operable to receive thecurrents from the first and second paths and allow the currents to flowto the second potential.
 5. The amplifier circuit of claim 4 wherein thefifth CMOS transistor is biased to allow a regulated current to flowthrough the fifth CMOS transistor.
 6. The amplifier circuit of claim 4wherein the first and the third CMOS transistors comprise PMOS-typetransistors.
 7. The amplifier circuit of claim 4 wherein the second,fourth and fifth CMOS transistors comprise NMOS-type transistors.
 8. Theamplifier circuit of claim 4 wherein the input capacitor is connectedbetween the amplifier input terminal and a gate of the secondtransistor, and wherein a second feedback capacitor is connected betweena drain of the fourth transistor and the gate of the second transistor.9. The amplifier circuit of claim 4 wherein the first stage amplifiercomprises a positive feedback transistor coupled in parallel with thesecond transistor, the positive feedback transistor having a gatecoupled to the drain of the fourth transistor.
 10. The amplifier circuitof claim 4 wherein the first and third CMOS transistors are configuredto as a current mirror so that the magnitude of the current flowingthrough the first current path is equal to the current flowing throughthe second current path.
 11. The amplifier circuit of claim 4 whereinthe first potential comprises a power supply voltage.
 12. The amplifiercircuit of claim 4 wherein the second potential comprises groundpotential.
 13. The amplifier circuit of claim 4 wherein the first biasvoltage comprises ground potential.
 14. The amplifier circuit of claim 1wherein the second stage amplifier comprises a unity gain invertingamplifier.
 15. A switched capacitor amplifier, comprising: an amplifiercircuit having a non-inverting input terminal and an output terminal; aninput capacitor connected between an amplifier input terminal and thenon-inverting input terminal of the amplifier circuit; a feedbackcapacitor connected between the output terminal of the amplifier circuitand the non-inverting input terminal of the amplifier circuit, thefeedback capacitor being operable to provide positive feedback to theamplifier circuit; and a set of switches operable to periodicallydischarge the input capacitor and the feedback capacitor.
 16. Theamplifier of claim 15 wherein the amplifier circuit comprises: first andsecond CMOS transistors coupled to form a first path for a current toflow from a first potential, the second CMOS transistor having a gatecoupled to the non-inverting input terminal of the first stageamplifier; third and fourth CMOS transistors coupled to form a secondpath for a current to flow from the first potential, the fourth CMOStransistor having a gate coupled to the inverting input terminal of thefirst stage amplifier, the first and third CMOS transistors beingcoupled together at their respective gates, the feedback capacitor beingcoupled between the drain of the fourth CMOS transistor and the gate ofthe second CMOS transistor; and a fifth CMOS transistor coupled to thesecond and fourth CMOS transistors and operable to receive the currentsfrom the first and second paths and allow the currents to flow to thesecond potential.
 17. The amplifier of claim 16 wherein the fifth CMOStransistor is biased to allow a regulated current to flow through thefifth CMOS transistor.
 18. The amplifier of claim 16 wherein the firstand the third CMOS transistors comprise PMOS-type transistors.
 19. Theamplifier of claim 16 wherein the second, fourth and fifth CMOStransistors comprise NMOS-type transistors.
 20. The amplifier of claim16 wherein the first and third CMOS transistors are configured to as acurrent mirror so that the magnitude of the current flowing through thefirst current path is equal to the current flowing through the secondcurrent path.
 21. The amplifier of claim 16 wherein the first potentialcomprises a power supply voltage.
 22. The amplifier of claim 16 whereinthe second potential comprises ground potential.
 23. A switchedcapacitor amplifier, comprising: first and second CMOS transistorshaving respective gates, the first and second CMOS transistors beingcoupled to each other to form a first path for a current to flow from afirst potential; an input capacitor connected between an amplifier inputterminal and the gate of the second CMOS transistor; third and a fourthCMOS transistors having respective gates, the third and fourth CMOStransistors being coupled to each other to form a second path for acurrent to flow from the first potential, the first and third CMOStransistors being coupled together at their respective gates; a fifthCMOS transistor coupled to the second and fourth CMOS transistors andoperable to receive the currents from the first and second paths andallow the currents to flow to the second potential; a positive feedbacktransistor coupled in parallel with the second CMOS transistor, thepositive feedback transistor having a gate coupled to the drain of thefourth CMOS transistor; and a switch for periodically discharging theinput capacitor.
 24. The amplifier of claim 23 wherein the fifth CMOStransistor is biased to allow a regulated current to flow through thefifth CMOS transistor.
 25. The amplifier of claim 23 wherein the firstand the third CMOS transistors comprise PMOS-type transistors.
 26. Theamplifier of claim 23 wherein the second, fourth and fifth CMOStransistors comprise NMOS-type transistors.
 27. The amplifier of claim23 wherein the first and third CMOS transistors are configured to as acurrent mirror so that the magnitude of the current flowing through thefirst current path is equal to the current flowing through the secondcurrent path.
 28. The amplifier of claim 23 wherein the first potentialcomprises a power supply voltage.
 29. The amplifier of claim 23 whereinthe second potential comprises ground potential.
 30. An operationalamplifier circuit, comprising: a first stage amplifier having an inputterminal and a non-inverting output terminal; an input capacitor havinga first terminal coupled to the input terminal of the first stageamplifier and an input terminal of the operational amplifier circuit; afirst feedback capacitor having a first terminal coupled to thenon-inverting output terminal and a second terminal coupled to the inputterminal of the first stage amplifier, the first feedback capacitorproviding positive feedback from the non-inverting output terminal tothe input terminal of the first stage amplifier; a second stageamplifier having an input terminal coupled to the non-inverting outputterminal of the first stage amplifier, the second stage amplifier havingan inverting output terminal; and a second feedback capacitor having afirst terminal coupled to the inverting output terminal of the secondstage amplifier and a second terminal coupled to the input terminal ofthe first stage amplifier, the second feedback capacitor providingnegative feedback from the inverting output terminal of the second stageamplifier to the input terminal of the first stage amplifier.
 31. Theoperational amplifier circuit of claim 30, further comprising a set ofswitches operable to periodically discharge the input capacitor and thesecond feedback capacitor.
 32. The operational amplifier circuit ofclaim 30 wherein the first stage amplifier comprises a plurality of CMOStransistors configured to form a first and a second current path, theCMOS transistors being operable to pass current through the first andsecond paths from a first potential to a second potential.
 33. Theoperational amplifier circuit of claim 32 wherein the plurality of CMOStransistors comprise a CMOS transistor providing a third path coupled tothe first and second paths.
 34. The operational amplifier circuit ofclaim 32 wherein the plurality of CMOS transistors configured to formthe first path comprise a NMOS-type transistor and a PMOS-typetransistor, and the plurality of CMOS transistors configured to form thesecond path comprise a NMOS-type transistor and a PMOS type transistor.35. A method of amplifying an input signal, comprising: providing afirst CMOS amplifier having relatively low open-loop gain, the firstCMOS amplifier having an output terminal and an input terminal receivingthe input signal; connecting a second CMOS amplifier in series with thefirst CMOS amplifier, the second CMOS amplifier having relatively lowopen-loop gain, the first and second CMOS amplifiers together invertingthe input signal to provide an inverted output signal, the first CMOSamplifier having an output terminal on which the output signal isprovided; causing the first CMOS amplifier to operate with positivefeedback, thereby substantially increasing the gain of the first CMOSamplifier; and connecting a capacitor between the output terminal of thesecond CMOS amplifier and the input terminal of the first CMOSamplifier.
 36. The method of claim 36 wherein the first CMOS amplifiercomprises a non-inverting amplifier, and wherein the act of causing thefirst CMOS amplifier to operate with positive feedback comprisescoupling the output terminal to the input terminal.
 37. The method ofclaim 36 wherein the act of coupling the output terminal to the inputterminal comprises coupling the output terminal to the input terminalthrough a capacitor.
 38. The method of claim 36 wherein the first CMOSamplifier includes a CMOS input transistor having a gate receiving theinput signal, and wherein the act of causing the first CMOS amplifier tooperate with positive feedback comprises coupling a second transistor inparallel with the input transistor and coupling a gate of the inputtransistor to the output terminal of the first CMOS amplifier.